In August 2006, a federal district court held that the Terrorist Surveillance Program violates the Fourth Amendment. Scholars have debated the legality and constitutionality of the program extensively since the New York Times first publicized its existence in December 2005. In this Article, I look beneath the surface of that raging debate to one of the premises underlying the court's conclusion, that the Fourth Amendment protects the confidentiality of communications. I explore the origins of the notion that the Fourth Amendment protects communications privacy. Most scholars and commentators look to Justice Brandeis's famous dissent in the 1928 case Olmstead v. United States. In this Article, I contend that we must go further back, back to surveillance of the first communications network in the United States, the post office. I explain the history of postal surveillance and show that the principle of communications privacy derives not from the Fourth Amendment or even from the Constitution at all. Rather, it comes from early postal policymakers who put that principle into postal ordinances and statutes in the late eighteenth century. Over time, the principle of communications privacy became embedded into the postal network by both law and custom. It was only then that the Court incorporated it into the Fourth Amendment in the 1878 case Ex parte Jackson, which in turn served as one of the bases of Justice Brandeis's Olmstead dissent. So, if today we see the principle of communications privacy as fundamental to the Fourth Amendment, we have postal policymakers to thank, for it was through the post office, not the Constitution or the Bill of Rights, that early Americans first established that principle. I conclude the Article with thoughts on the implications of this story for constitutional theory and, in particular, the relationship between the courts, the legislature, and institutions in the creation of constitutional law doctrine.